Electric cable having improved resistance to moisture

ABSTRACT

An electric cable having a cylindrical protective metal shield formed by overlapping the edges of a metal strip to form a longitudinal seam and an outer plastic jacket is rendered resistant to moisture by means of a sheath in the form of a layer of a hygroscopic material such as paper or a blend of a polymer and a siccative drying agent and a metal shield surrounding the layer of hygroscopic material and having the edges thereof in overlapping relationship to form a longitudinal seam bonded together by means of an adhesive polymer. The sheath is disposed within the cable between the protective metal shield and the outer plastic jacket such that the layer of hygroscopic material is in contact with the protective metal shield and the metal shield of the sheath is in contact with the outer plastic jacket.

United States Patent [72] Inventor Raymond C. Mildner Midland, Mich.[21] Appl. No. 799,674 [22 Filed Feb. 17, 1969 [45] Patented June 29,1971 [73] Assignee The Dow Chemical Company Midland, Mich.

[541 ELECTRIC CABLE HAVING IMPROVED RESISTANCE T0 MOISTURE 18 Claims, 3Drawing Figs, [52] US. Cl 174/105, 174/28,174/36, 174/107, 174/113 [511'Int. Cl. H0lb7/28 [50] Field of Search 174/26- -29, 36,34,102,105,107,1l3,1l0.4, 110.8, 113, 116, 121.1, 121 5 6] ReferencesCited UNITED STATES PATENTS 2,056,085 9/1936 Alles 174/105 2,186,7931/1940 Wodtke 174/116 3,211,821 10/1965 Wakefield 174/26 3,272,91 19/1966 Rollins et a]. 174/106 3,315,025 4/1967 Tomlinson 174/1073,328,514 6/1967 Cogelia 174/113 3,332,138 7/1967 Garner ..l14/SHIELDDIGEST 3,365,534 l/1968 Volk 174/29 FOREIGN PATENTS 734,163 7/1955 GreatBritain 174/28 Primary Examiner- Lewis I-I. Myers Assistant Examiner-A.T, Grimley Attorneys-Griswold and Burdick, Richard G. Waterman,

Lester .I. Dankert and Ralph M. Mellom ing the edges thereof inoverlapping relationship to form a lon-.

gitudinal seam bonded together by means of an adhesive polymer. Thesheath is disposed within the cable between the protective metal shieldand the outer plastic jacket such that the layer of hygroscopic materialis in contact with the protective metal shield and the metal shield ofthe sheath is in contact with the outer plastic jacket.

Re /u/'n conduc /o/ ELECTRIC CABLE HAVING IMPROVED RESISTANCE T MOISTUREThis invention relates to electric cables. In one aspect, this inventionrelates to electric cables having improved resistance to moisture. Inanother aspect, this invention relates to coaxial and twisted paircables wherein the insulating medium between the conductors is protectedagainst the penetration of moisture into the cables.

The efficiency and indeed the ability of a cable to effectively transmitsignals within the audio and video frequency ranges and to transmitelectric current in power distribution systems is dependent to a largeextent upon the condition of the cable. While many factors affect thecondition of a cable when it is in service, the humidity or amount ofmoisture within the cable is probably the single most important factorgoverning the ability of a cable to effectively transmit electricsignals. Since many cables are used in areas of high humidity such aswhen they are buried in the earth near or below the water table or whenthey are used above ground and thus subjected to rain and snow, it isnecessary to construct cables which are impermeable to water and otherforms of moisture.

The advent of outer plastic jackets on cables has contributedsubstantially to preventing water from permeating into the cable.Although the outer plastic jacket appears impervious to moisture, suchis not the case as evidenced by the presence of moisture within thecable determined by actual measurement.

More recent developments in cable construction employ a cylindricalprotective metal shield interiorally of the plastic jacket to assist inpreventing moisture from permeating into the working parts of the cable.The cylindrical metal shield is formed from a strip of metal foldedaround the cable core such that the edges overlap to form a'longitudinal seam which is bonded together with an adhesive polymer.Although the protective metal shield does reduce moisture permeationinto the cable, and is probably sufficient in some types of cables suchas those designed to transmit signals at low (audio) frequencies theadhesive polymer used to bond the longitudinal seam is not imperviousand will permit minute quantities of moisture to pass into the cablewhere it will collect in the insulation between the conductors. When theinsulation is of a type sensitive to moisture, such as a foamed plasticmaterial, the presence of these minute quantities of moisture canproduce disastrous results. And, while the moisture may be in vapor forminitially, a decrease in ambient temperature will cause the vapor tocondense in the insulation and produce droplets of water which cancompletely destroy the signal being transmitted through the conductors.While hermetic sheaths such as those fashioned from lead or aluminum canbe employed to prevent moisture permeation in place of the pro tectivemetal shield having the longitudinal seam, this type of protection hasmany disadvantages in that it is much more expensive, the resultingcable is much heavier, and the cable loses its flexibility.

While it is of course desirable to maintain the interior of all types ofcables free of moisture, it is particularly important to keep theinterior of air spaced or plastic foam insulated coaxial and twistedpair cables dry since even minute quantities of moisture within theelectric field between the conductors in these types of cables reducessubstantially the efiiciency of the signals being transmitted andfrequently the moisture will shunt the signal across the conductorsthereby causing complete failure. Moreover, if the relative humidity ofthe air within the cable increases by the permeation of even minutequantities of moisture, a small drop in ambient temperature can causecondensation of the moisture within the cable thus producing disastrousresults on the electrical performance of the cable.

According to this invention, the problems incident to the unavoidablepenetration of moisture through the outer plastic jacket and protectivemetal shield of a cable are overcome by means of a sheath comprising, incombination, a layer of hygroscopic material disposed around theprotective metal shield interiorly of the outer jacket to serve as asink for absorbing moisture and a metal shield surrounding the layer ofhygroscopic material and disposed between the hygroscopic material andthe outer jacket. The cable construction of this invention thus has theprotective metal shield located as an inner shield relative to thesheath and the outer plastic jacket. Similarily, the metal shield of thesheath is located as an outer shield relative to the inner protectivemetal shield. The hygroscopic layer and the metal shield surrounding thehygroscopic layer function in combination to prevent moisture frommigrating into the working parts of the cable.

The metal shield between the hygroscopic layer and the outer jacket isformed by longitudinally folding a metal strip such that the edgesthereof are in overlapping relationship to form a longitudinal seamwhich is bonded together by means of an adhesive polymer. An importantrequirement of the sheath of this invention is that the metal shieldelement of the sheath be reasonably resistant to the permeation ofmoisture to obviate the necessity of having to use a very thickhygroscopic layer which would otherwise be necessary to accommodatelarge quantities of moisture. The sheath of this invention is thusdistinguishable from the concept of employing concentric plastic layerswith a layer of moisture absorbent material between them. In the latterconstruction, the layer of absorbent material would have to be ofconsiderable thickness in order to accommodate the large quantities ofmoisture which would pass through the outer plastic layer of theconcentric plastic layers.

Accordingly, it is the object of this invention to provide an electriccable of improved resistance to moisture.

Another object of this invention is to provide a layer of hygroscopicmaterial in a cable 'to serve as a sink for absorbing moisture.

A further object of this invention is to provide a cable which performssatisfactorily during exposure to moisture.

These and other objects of the invention will become apparent to oneskilled in the art after studying the following detailed description,the appended claims, and the accompanying drawings wherein:

FIG. I is a partial section in isometric view of a cable constructedaccording to one embodiment ofthe invention;

FIG. 2 is a cross section of a cable constructed according to anotherembodiment of the invention; and

FIG. 3 is a cross section of a cable constructed according to yetanother embodiment of the invention.

While the cables illustrated in the drawings are intended to showspecific cable constructions embodying the novel concept of theinvention, the drawings are for illustration purposes only and manyother cable constructions within the spirit and scope of the inventionare possible. Similarily, the cables illustrated in the drawings are notnecessarily drawn to scale nor do they necessarily illustrate therelative size of the several elements.

A cable of the kind with this invention is concerned is one whichcomprises at least one conductor for transmitting electric signals,insulating means surrounding the conductor, a protective metal shieldsurrounding the insulating means, and an outer jacket of plasticmaterial. The objects of this invention are realized in the cabledescribed above by the improvement which comprises a sheath comprising,the combination of, a layer of hygroscopic material disposed around theprotective metal shield interiorly of the outer jacket to serve as asink for absorbing moisture and a metal shield surrounding the layer ofhygroscopic material and disposed between the layer of hygroscopicmaterial and the outer plastic jacket. The metal shield surrounding thelayer of hygroscopic material is fashioned such that the edges thereoflie in overlapping relationship to form a longitudinal seam which isbonded together by means of an adhesive polymer. In a modification ofthe cable constructions described above, a layer of plastic materialsuch as a polyolefin or the like is disposed between the protectivemetal shield surrounding the insulating means and the layer ofhygroscopic material.

Any hygroscopic material suitable to serve as a sink for absorbingmoisture which migrates'through the outer plastic jacket and whichmigrates to a much less degree through the longitudinal seam of themetal shield can be used in the practice of the invention. Exemplaryhygroscopic materials which can be used include paper, cloth, and blendsof polymer material and compatible siccative drying agents such ascalcium chloride, sodium sulfate, sodium chloride, and the like. Paperis generally preferred as the hygroscopic material because it isgenerally inert to moisture, relatively inexpensive, and freelyavailable. Further, the paper is quite porous and thus can accommodatethe moisture by swelling without imposing dangerous stresses on thecable. Any suitable polymer having a comparatively low modulus which iscapable of accommodatinglarge amounts of the siccative drying agentwithout becoming brittle can be used in the practice of the invention.Exemplary polymer materials which can be blended with the siccativedrying agent to form the hygroscopic material includes chlorinatedpolyethylene, copolymers of ethylene and isobutyl acrylate, and thelike. Since a corrosive environment is likely to occur when thesiccative drying agent absorbs moisture, a suitable corrosion inhibitorcan be admixed with the polymer material and the siccative drying agentif desired.

While the amount of siccative drying agent blended with the Polymermaterial is largely a matter of personal choice dictated by such factorsas economics and the like, it is generally preferred that the dryingagent by employed in an amount between about and about 80 weight percentbased upon the total weight of the blend.

The metal shield surrounding the layer of hygroscopic material whichfunctions in combination with the hygroscopic material to improve theresistance to moisture of the cables constructed in accordance with thisinvention is fashioned of any suitable metal such as, for example,aluminum, copper, bronze, steel, composites of two or more of theforegoing metals, and the like. The metal shield can be of any suitableand convenient thickness such as, for example, between about 2 and about20 or more mils.

It is generally preferred that the metal shield be adhesively bonded tothe outer jacket by means of an adhesive disposed as a layer oversubstantially the entire area of contact between the metal shield andthe outer jacket. In that embodiment of the invention wherein a layer ofplastic material is disposed between the protective metal shield and thelayer of hygroscopic material, it is generally preferred that theprotective metal shield be bonded to the plastic layer by means of anadhesive disposed as a layer over substantially the entire area ofcontact between them. It is also evident that the optional layer ofplastic material can contain the siccative drying agent and thus serveas the hygroscopic layer if desired.

Any suitable adhesive can be used to bond the several elements of thecables together. Exemplary adhesives include polymers of an olefin suchas, for example, ethylene, propylene, and the like, and an ethylenicallyunsaturated carboxylic acid having between 3 and 8 carbon atoms permolecule such as, for example, acrylic acid, methacrylic acid,ethacrylic acid, crotonic acid, isocrotonic acid, tiglic acid, angelicacid, senecioic acid, and the like.

Both random and graft copolymers of the olefin and the ethylenicallyunsaturated carboxylic acid can be used in the practice of theinvention. These adhesive copolymers can be obtained commercially orthey can be prepared by processes well known in the art. While theinvention is not to be bound by any particular technique for preparingthe adhesive copolymers, one exemplary technique for producing randomcopolymers, one exemplary technique for producing random copolymersinvolves subjecting a mixture of the olefin monomers and acid monomersto a high pressure such as between about 500 and about 1000 atmospheresand to an elevated temperature such as between about 100 and about 400C. in the presence of a suitable free radical initiator such asditertiary butyl peroxide. Reaction conditions can be varied to producerandom copolymers having the desired molecular weight. Exemplarytechniques for producing the graft copolymers which can beused in thepractice of this invention are outlined in US. Pat. Nos. 3,177,269 and3,270,090, the disclosures of which are both specifically incorporatedherein by reference.

Referring now to the drawings, wherein like reference numerals are usedto denote like elements whenever convenient the invention will bedescribed in more detail in connection with several embodimentsillustrative of the invention.

FIG. 1, a coaxial cable indicated generally by reference numeral 1comprises a conductor 2 of copper or the like disposed substantially inthe center of the cable and insulating means comprising a plurality ofdisc-shaped spacers 3 transverse to the conductor 2 and longitudinallyspaced thereon. A protective metal shield 4 fashioned of copper,aluminum, or the like and having a longitudinal seam 5 formed byoverlapping the edges of a metal strip surrounds the insulating means.In the coaxial cable illustrate in FIG. 1, protective metal shield 4serves as a return conductor. A plastic layer 7 of a polyolefin such aspolyethylene, polypropylene, or the like surrounds the protective metalshield 4 and is optically bonded thereto by means of an adhesive layer 8applied to the outer surface of the protective metal shield 4. Theplastic layer 7 is an optional feature of the cable constructionillustrated in FIG. 1 and the coaxial cable 1 can be fabricated withoutit if desired.

A layer 9 of hygroscopic material fashioned of, for example, paper,textile cloth, blend of polymer material and siccative drying agent, orthe like is disposed around the plastic layer 7 and can optionally bebonded thereto by means of an adhesive layer 11 coated on the outersurface of the plastic layer 7. A metal shield 12 surrounds the layer 9of hygroscopic material and is formed such that the edges thereof lie inoverlapping relationship to form a longitudinal seam 13 which is bondedtogether by means of an adhesive polymer disposed as a bead or the likealong the edges of the metal shield. The metal shield 12 is fashioned ofany suitable metal such as, for example, aluminum, copper, bronze,steel, composites of two or more of the foregoing metals, or the like.

An outer plastic jacket 14 of a polyolefin material such aspolyethylene, polypropylene, or the like surrounds the metal shield 12and is preferably bonded thereto by means of an adhesive polymer layer16 applied to the outer surface of the metal shield.

The cable illustrated in FIG. 1 can be fabricated by suitable techniqueswell known in the art. The disc-shaped spacers 3 are provided withradial slits (not shown) which allow then to be mounted on the conductor2 as the conductor passes into a cable fabrication apparatus. Theprotective metal shield 4 surrounding the spacers 3 is formed bylongitudinally folding a metal strip and overlapping the edges thereofto provide the longitudinal seam 5 which is bonded together by anadhesive. The optional plastic layer 7 is then extruded over the metalshield 4 by passing the same through a conventional extrusion apparatus.The layer 9 of hygroscopic material is positioned around the plasticlayer 7 by any suitable technique depending upon the nature of thehygroscopic material. When paper or a textile cloth is employed as thehygroscopic material, it is hellcally wrapped or longitudinally foldedover the plastic layer 7. When the hygroscopic material comprises apolymer blend and a siccative drying agent it is disposed around theplastic layer 7 by extrusion or the like. The metal shield 12 isdisposed around the layer 9 of hygroscopic material by longitudinallyfolding a metal strip to provide the longitudinal seam 13. The outerplastic jacket 14 is then extruded upon the metal shield 12 by suitableextrusion apparatus.

When the protective metal shield 4 and the metal shield 12 are providedwith adhesive coatings to form a strong bond with their respectiveadjacent elements in the cable, such adhesive coatings can be applied byextruding the adhesive onto the metal surfaces, depositing the adhesivefrom solution or a latex, film lamination, or the like.

While the disc-shaped spacers 3 are illustrated as the insulation meansin FIG. 1, it is evident that other forms of insulation can be used suchas a helical strip of insulating material or a foamed polyolefin such asfoamed polyethylene, foamed polypropylene, or the like. When a foamedpolyolefin is employed as the insulation means, it is preferable appliedby passing the conductor 2 through an extruder which extrudes thepolyolefin containing a blowing agent under conditions of temperatureand pressure to allow the blowing agent toexpand the polyolefin uponleaving the extruder and thereby form a cellular layer.

The protective metal shield 4 in the cable structure illustrated in FIG.l is generally fashioned of a material such as copper or the like sothat it serves as a conductor for transmitting electric signals.

FIG. 2 of the drawings illustrates a cable shown generally by referencenumeral 21 comprising two coaxial cables shown generally by referencenumerals 22 and 23 which are arranged in a generally parallelrelationship. Each of the coaxial cables 22 and 23 comprises a firstconductor 24 for transmitting electric signals, insulating means 26surrounding the first conductor 24, a second or return conductor 27 in aform ofa cylinder positioned around the insulating means 26 with theedges thereof in overlapping relationship to form a longitudinal seam 28which is preferably bonded together by means of an adhesive, and anouter jacket 29 of plastic material surrounding the conductor 27.Conductor 24 is coaxial cables 22 and 23 is preferably fashioned ofcopper or the like. The insulation means 26 comprises a foamedpolyolefin such as foamed polyethylene or the like. The insulating meanscan optionally comprise the disc-shaped spacers 3 described inconnection with FIG. 1 of the drawings. The return conductor 27 in eachof the coaxial cables 22 and 23 is preferably fashioned of a highlyconductive material such as copper or the like. The outer plastic jacket29 in each of the coaxial cables is a suitable polyolefin such as, forexample, polyethylene.

An optional layer 31 of plastic material which can be either unfoamed orfoamed polyethylene or the like is disposed between the coaxial cables22 and 23 and a layer 9 of hygroscopic material. A metal shield 12surrounding the layer of hygroscopic material has the edges thereof inoverlapping relationship to form a longitudinal seam 13 which is bondedtogether by means of an adhesive polymer. An outer jacket M of asuitable plastic material such as a polyolefin or the like surrounds themetal shield 12.

As described in connection with FIG. 1 of the drawings, the severalelements of the cable illustrated in FIG. 2 can be bonded together bymeans of an adhesive copolymer of an olefin and an ethylenicallyunsaturated carboxylic acid. Thus, the return conductor 27 is optionallybonded to the insulation means 26 and the outer jacket 29. Similarly,the metal shield 12 is optionally adhesively bonded to the outer jacket14 by means of the adhesive polymer.

While the cable 21 of FIG. 2 illustrates two coaxial cables 22 and 23disposed within the hygroscopic layer 9 and the metal shield 12, it iswithin the spirit and scope of the invention to employ a plurality ofcoaxial cables arranged in a generally parallel relationship. The layer9 of the hygroscopic material, the metal shield 12, and the remainingelements of cable 21 are fabricated of the same materials described inconnection with the cable illustrated in FIG. l. Cable 21 is fabricatedby employing well known cable forming apparatus including cable shieldforming means, cable jacket extruders, and the like.

The cable design shown by FIG. 3 of the drawing is illustrative of atwisted pair cable employing the inventive concept of the invention. Atwisted pair cable shown generally by reference numeral 32 in FIG. 3comprises a pair of conductor 33 individually insulated with a layer 34of foamed polyolefin such as polyethylene or the like which are twistedtogether to form a twisted pair having opposed helical valleys definedby the outer surfaces of the layers 34. A layer 36 of a foamedpolyolefin such as polyethylene surrounds the twisted pair and isdisposed substantially throughout the helical valleys defined by layers34 to provide a cylindrical cable core of foamed insulation. A.protective metal shield 37 surrounds the layer 36 and has the edgesthereof in overlapping relationship to form a longitudinal seam 33. Theedges which form seam 38 are preferably bonded together by means of anadhesive polymer. A layer 39 of solid plastic material such as apolyolefin is disposed between the protective metal shield 37 and alayer 9 of hygroscopic material. The plastic layer 39 is optional andcan be omitted from the cable construction illustrated in FIG. 3 isdesired. When the layer 39 of plastic material is not used, the layer 9of hygroscopic material is in direct contact with the protective metalshield 37.

A metal shield 12 surrounds the layer 9 of hygroscopic material and isgenerally coextensive therewith. The metal shield 12 is formed with theedges thereofin overlapping relationship to form a longitudinal seam 13which is bonded together by means of the adhesive copolymer. The metalshield 12 is fashioned ofa suitable metal such as, for example,aluminum, copper, bronze, steel, composites of two or more of theforegoing metals, or the like. An outer jacket 14 of plastic materialsurrounds the metal shield 12 and is fashioned of a suitable polymermaterial such as polyethylene, poly (vinylchloride), or the like.

The layer 9 of hygroscopic material is fashioned from any suitablematerial such as, for example, those materials described in connectionwith FIGS. 1 and 2 of the drawings. To add mechanical strength to thecable of FIG 3, it is generally preferred to employ an adhesivecomprising a copolymer of an olefin and an ethylenically unsaturatedcarboxylic acid disposed as a layer over substantially the entire areaof contact between layer 36 of foamed polyolefin and protective metalshield 37 and over substantially the entire area of contact betweenmetal shield 12 and outer jacket 14. While the cable design illustratedby FIG. 3 is a twisted pair cable, it is within the spirit and scope ofthe invention to employ any number of twisted pairs or twisted quads.

In cables employing the novel concept of this invention, it is generallypreferred that some free space be provided between the protective metalshield and the metal shield surrounding the layer of hygroscopicmaterial to allow the hygroscopic material to expand as an incident tothe absorption of water. As previously indicated, paper in and of itselfnormally contains enough free space to accommodate this expansion. Whenthe hygroscopic layer is fashioned of a blend of a polymer material anda siccative drying agent, the polymer material can be partially orcompletely foamed if desired. However, longitudinal passages in thevicinity of the hygroscopic layer should be avoided to preventlongitudinal movement ofthe moisture.

Although the invention has been described primarily in connection withcoaxial cables, this has been done because problems associated withmoisture penetration are more severe in these type of cables. It isevident that the invention is applicable to all types of cables andparticularly those where moisture is a problem.

While, as previously indicated, the metal shield of the sheath can beconstructed of any suitable metal, it is generally preferred to employtin plated steel because of the better electromagnetic shielding whichthis material provides at low frequencies and because it is mechanicallystronger than the softer metals such as copper or aluminum, so that itaffords better mechanical protection to the cable.

The following Example illustrates the improved results obtained with thecable sheath of this invention. It must be understood that this Exampleis for illustration purposes only and should not be construed aslimiting of the invention.

EXAMPLE An air-spaced coaxial cable having an insulated centerconductor, a return conductor, and a layer ofpolyethylene in the form ofan inner jacket surrounding the return conductor is provided with asheath comprising a layer of hygroscopic paper 20 mils thick and a metalshield surrounding the hygroscopic paper layer. The metal shield isformed by longitudinally folding an 8-mil thick strip of aluminum aroundthe layer of hygroscopic paper such that the edges of the strip overlapto form a longitudinal seam. The strip of aluminum which forms the metalshield has a 2-mil thick coating of an adhesive copolymer of ethyleneand acrylic acid on the surface of the strip which becomes the exteriorsurface of the metal shield. The adhesive copolymer bonds the edges ofthe metal shield along the longitudinal seam and also bonds the metalshield to an outer jacket of polyethylene which is then extruded overthe shield. The several elements of the cable are set forth in Table 1below.

In the following calculations, which illustrate the resistance tomoisture of a cable having a sheath constructed in accordance with thisinvention, the permeability (P) of low density polyethylene is taken asl l cc. at standard temperature and pressure per centimeter per secondper cm. of Hg. at C. The calculations also employ an upper limit 50percent relative humidity in the air space between the conductors. Thisupper limit may be accepted as the maximum tolerable relative humidityto avoid condensation within the air space when the temperature of thecable decreases.

The flow of moisture, F, through a layer of polyethylene in a cable,such as through the inner jacket or the outer jacket, is governed by theexpression where: F cc of water/day/cm. of cable length P permeabilityof the polyethylene A p pressure differential across the polyethylenelayer D Outer diameter of the polyethylene layer D inner diameter of thepolyethylene layer in the several runs, the cable is immersed in waterto provide an ambient relative humidity of 100 percent.

Control No. l

The flow of moisture (F) into a coaxial cable comprising only Elements1, 2 and 3 of Table 1, except that the longitudinal seam of the returnconductor is not bonded nor is the return conductor bonded to the innerjacket (i.e., the return conductor does not have an adhesive copolymercoating), is determined by the formula above to be 2.4 cc. per day percm. of cable length. This corresponds to about 40 micrograms per dayper'cm. of cable length. The volume of air between the conductors isabout 2 cubic centimeters per centimeter length of cable. Since thedensity of saturated water vapor is 17.3 micrograms /cc. at 20 C., theair space between the conductors would have a relative humidity ofpercent in less than one day for a water vapor flow of 40 micrograms perday. Control No. 2

Experience has shown that by adhesively bonding the longitudinal seam inthe return conductor and by bonding the return conductor to the jacketofa coaxial cable, the moisture penetration through the jacket and theseam in the return conductor is improved by a factor of about 100. Thus,the flow of moisture (F) into a coaxial cable comprising only elements1, 2, and 3 of Table l and having the longitudinal seam bonded with theadhesive copolymer and the return conductor bonded to the inner jacketis reduced to about 0.4 micrograms per day per cm. of cable length. Therelative humidity RH within this cable will increase about 1.16 percentper day as determined by the expression The time, T, required to reach arelative humidity of 50 percent is about 60 days as determined by theexpression.

100 100 T I175 It is evident that a cable of the type described havingonly 60 days of satisfactory operation is unsuitable. Control No. 3

A coaxial cable is made from elements 1, 2, 3, 5, and 6 only of Table 1except no adhesive copolymer is employed in the construction (i.e., thesame cable construction as outlined in control No. 1 except in this runthe cable is provided also with an unbonded rnetal shield and an outerjacket). In this construction, the additional metal shield and outerjacket would serve to reduce moisture penetration by one-half. The airspace between the conductors in this construction would achieve about100 percent relative humidity in about 2 days. Control No. 4

A coaxial cable is made from the elements in control 3 except thelongitudinal seams in the return conductor and in the metal shield arebonded with adhesive copolymer. The return conductor and metal shieldare also adhesively bonded to their respective polyethylene elements. Inthis construction, the additional metal shield and outer jacket servesto reduce the penetration of moisture over the cable construction ofControl No. 3 by one-half. Thus, the air in the air space of this cableconstruction will reach a relative humidity of 50 percent in about days.

=60 days.

CABLE CONSTRUCTION OF TABLE I A cable constructed according to theinvention and employing all of the elements identified in Table lutilizes a hygroscopic paper weighing about 0.3 grams per cm. of cablelength. The paper is of a type which will absorb water vapor equal toabout 7.5 weight percent of its own weight when in equilibrium with airhaving a relative humidity of 50 percent or about 15 weight percent whenin equilibrium with air havi-ng a relative humidity of 100 percent.Since a cable construction having a bonded metal shield and a bondedjacket will allow about 0.4 micrograms of water vapor to enter thehygroscopic paper per day, when the cable is submerged in water therelative humidity, RH of the hygroscopic paper will increase about 9 l0percent per day as determined by the expression cent in the hygroscopiclayer is about 7.7 l0" days (about 2 10 years) as detennined by theexpression X 100 =about 9 X 10* (approx.

100 log =7.7 10 days 2(0 years) f Control If the longitudinal seam ofthe metal shield and the return conductor were left unbonded and themetal shield and return conductor were not bonded to their respectiveplastic ele ments, the time required to reach a relative humidity of 50percent in the Cable Construction of Table I set forth above would bereduced by a factor of 100 (as related in Control No. 2) thus giving anoperational life of only 770 days or just slightly more than 2 years. itis thus evident that the improvement of this invention is attributed tothe combination of an adhesively bonded metal shield and the layer ofhygroscopic material.

While the invention has been described in considerably detail, it mustbe understood that such description is for that purpose only and shouldnot be construed as limiting of the invention.

What I claim is:

1. In a cable comprising at least one conductor for transmittingelectric signals, insulating means surrounding said conductor, aprotective metal shield surrounding said insulating means andcharacterized by having a longitudinal seam formed by overlapping theedges of a metal strip used to form said protective metal shield, and anouter jacket of plastic material, the improvement comprising a sheathcomprising, in combination:

A. a layer of hygroscopic material disposed around said protective metalshield interiorally of said outer jacket to serve as a sink forabsorbing moisture, and

B. a metal shield surrounding said layer of hygroscopic material anddisposed between said layer of hygroscopic material and said outerjacket, said metal shield having the edges thereof in overlappingrelationship to form a longitudinal seam bonded together by means of anadhesive polymer.

2. A cable according to claim 1 wherein said layer of hygroscopicmaterial is fashioned of a material selected from the group consistingof paper, cloth, blends of polymer material and calcium chloride, blendsof polymer material and sodium sulfate and blends of polymer materialand sodium chloride.

3. A cable according to claim 1 wherein said metal shield surroundingsaid layer of hygroscopic material is fashioned of a metal selected fromthe group consisting of aluminum, copper, bronze, steel, and compositesof two or more of the foregoing metals.

4. A cable according to claim 1 wherein said metal shield disposedbetween said layer of hygroscopic material and said outer jacket isadhesively bonded to said outer jacket by means of an adhesivecomprising a polymer of an olefin and an ethylenically unsaturatedcarboxylic acid.

5. A cable according to claim 1 wherein said conductor is a singleconductor of copper disposed substantially in the center of said cable,said insulating means comprises a plurality of disc-shaped spacerstransversely positioned on said conductor and longitudinally spacedthereon, and said protective metal shield comprises a conductorfashioned of a material selected from the group consisting of copper andaluminum.

6. A cable according to claim 1 which includes a layer of plasticmaterial disposed between said protective metal shield and said layer ofhygroscopic material.

7. A cable according to claim 6 wherein an adhesive comprising acopolymer of an olefin and an ethylenically unsaturated carboxylic acidis disposed as a layer over substantially the entire areas of contact a.between said protective metal shield and said layer of plastic materialwhich is disposed between said protective metal shield and said layer ofhygroscopic material, and

b. also between said metal shield, which is disposed between said layerof hygroscopic material and said outer jacket, and said outerjacket.

8. A cable according to claim 1 wherein said conductor is a singleconductor of copper disposed substantially in the center of said cable,said insulating means comprises a foamed polyolefin, and said protectivemetal shield comprises a con ductor fashioned of a material selectedfrom the group consisting of copper and aluminum.

jacket of plastic material,

9. A cable according to claim 8 wherein an adhesive comprising acopolymer of an olefin and an ethylenically unsaturated carboxylic acidis disposed as a layer over substantially the entire area of contactbetween said metal shield, which is disposed between said layer ofhygroscopic material and said outer jacket, and said outer jacket.

10. In a cable comprising a plurality of coaxial cables arranged in agenerally parallel relationship, each of said coaxial cables comprising;

a. a first conductor for transmitting electric signals,

b. insulating means surrounding said first conductor,

0. a second conductor in the form of a cylinder positioned around saidinsulating means characterized by having a longitudinal seam formed byoverlapping the edges of a metal strip used to form said secondconductor, and

d. an outer jacket of plastic material surrounding said secondconductor; and

an outer jacket of plastic material surrounding said plurality ofcoaxial cables, the improvement comprising a sheath comprising, incombination:

A. a layer of hygroscopic material surrounding said plurality of coaxialcable interiorally of said outer jacket and substantially coextensivewith said plurality of coaxial cables to serve as a sink for absorbingmoisture; and

B. a metal shield surrounding said layer of hygroscopic material anddisposed between said layer of hygroscopic material and said outerjacket, said metal shield having the edges thereof in overlappingrelationship to form a longitudinal seam bonded together by means of anadhesive.

11. A cable according to claim 10 which includes a layer of plasticmaterial disposed between said plurality of coaxial cables and saidlayer of hygroscopic material.

12. A cable according to claim 10 wherein an adhesive comprising acopolymer of an olefin and an ethylenically unsaturated carboxylic acidis disposed as a layer over substantially the entire areas of contact a.between said second conductor and said outer jacket in each of saidcoaxial cables, and

b. also between said metal shield and said outerjacket.

13, A cable according to claim 10 wherein a. said insulating meanssurrounding said first conductor in each of said coaxial cablescomprises a foamed polyolefin,

b. said second conductor in each of said coaxial cables is fashioned ofametal selected from the group consisting of aluminum and copper, and

c. said metal shield surrounding said layer of hygroscopic material isfashioned of a metal selected from the group consisting of aluminum,copper, bronze, steel, and composites of two or more of the foregoingmetals.

14. A cable according to claim 10 wherein said layer of hygroscopicmaterial is fashioned of a material selected from the group consistingof paper, cloth, blends of polymer material and calcium chloride, blendsof polymer material and sodium sulfate, and blends of polymer materialand sodium chloride.

15. ln a twisted pair cable comprising a pair of conductors individuallyinsulated with a layer of foamed polyolefin and twisted together to forma twisted pair having opposed helical valleys defined by the outersurfaces of said layers of foamed polyolefin, a layer of foamedpolyolefin surrounding said twisted pair and disposed substantiallythroughout the helical valleys therein, a protective metal shieldsurrounding said layer of foamed polyolefin and characterized by havinga longitudinal seam formed by overlapping the edges of a metal stripused to form said protective metal shield, and an outer the improvementcomprising a sheath comprising, in combination:

A. a layer of hygroscopic material disposed around said protective metalshield interiorally of said outer jacket and substantially coextensivewith said protective metal shield to serve as a sink for absorbingmoisture; and

B. a metal shield surrounding said layer of hygroscopic material anddisposed between said layer of hygroscopic material and said outerjacket, said metal shield having the edges thereof in overlappingrelationship to form a longitudinal seam bonded together by means of anadhesive.

16. A twisted pair cable according to claim 15 which includes a layer ofplastic material disposed between said protective metal shield and saidlayer of hygroscopic material.

17. A twisted pair cable according to claim 15 wherein said layer ofhygroscopic material is fashioned of a material selected from the groupconsisting of paper, cloth, blends of

1. In a cable comprising at least one conductor for transmittingelectric signals, insulating means surrounding said conductor, aprotective metal shield surrounding said insulating means andcharacterized by having a longitudinal seam formed by overlapping theedges of a metal strip used to form said protective metal shield, and anouter jacket of plastic material, the improvement comprising a sheathcomprising, in combination: A. a layer of hygroscopic material disposedaround said protective metal shield interiorally of said outer jacket toserve as a sink for absorbing moisture, and B. a metal shieldsurrounding said layer of hygroscopic material and disposed between saidlayer of hygroscopic material and said outer jacket, said metal shieldhaving the edges thereof in overlapping relationship to form alongitudinal seam bonded together by means of an adhesive polymer.
 2. Acable according to claim 1 wherein said layer of hygroscopic material isfashioned of a material selected from the group consisting of paper,cloth, blends of polymer material and calcium chloride, blends ofpolymer material and sodium sulfate and blends of polymer material andsodium chloride.
 3. A cable according to claim 1 wherein said metalshield surrounding said layer of hygroscopic material is fashioned of ametal selected from the group consisting of aluminum, copper, bronze,steel, and composites of two or more of the foregoing metals.
 4. A cableaccording to claim 1 wherein said metal shield disposed between saidlayer of hygroscopic material and said outer jacket is adhesively bondedto said outer jacket by means of an adhesive comprising a polymer of anolefin and an ethylenically unsaturated carboxylic acid.
 5. A cableaccording to claim 1 wherein said conductor is a single conductor ofcopper disposed substantially in the center of said cable, saidinsulating means comprises a plurality of disc-shaped spacerstransversely positioned on said conductor and longitudinally spacedthereon, and said protective metal shield comprises a conductorfashioned of a material selected from the group consisting of copper andaluminum.
 6. A cable according to claim 1 which includes a layer ofplastic material disposed between said protective metal shield and saidlayer of hygroscopic material.
 7. A cable according to claim 6 whereinan adhesive comprising a copolymer of an olefin and an ethylenicallyunsaturated carboxylic acid is disposed as a layer over substantiallythe entire areas of contact a. between said protective metal shield andsaid layer of plastic material which is disposed between said protectivemetal shield and said layer of hygroscopic material, and b. also betweensaid metal shield, which is disposed between said layer of hygroscopicmaterial and said outer jacket, and said outer jacket.
 8. A cableaccording to claim 1 wherein said conductor is a single conductor ofcopper disposed substantially in the center of said cable, saidinsulating means comprises a foamed polyolefin, and said protectivemetal shield comprises a conductor fashioned of a material selected fromthe group consisting of copper and aluminum.
 9. A cable according toclaim 8 wherein an adhesive comprising a copolymer of an olefin and anethylenically unsaturated carboxylic acid is disposed as a layer oversubstantially the entire area of contact between said metal shield,which is disposed between said layer of hygroscopic material and saidouter jacket, and said outer jacket.
 10. In a cable comprising aplurality of coaxial cables arranged in a generally parallelrelationship, each of said coaxial cables comprising; a. a firstconductor for transmitting electric signals, b. insulating meanssurrounding said first conductor, c. a second conductor in the form of acylinder positioned around said insulating means characterized by havinga longitudinal seam formed by overlapping the edges of a metal stripused to form said second conductor, and d. an outer jacket of plasticmaterial surrounding said second conductor; and an outer jacket ofplastic material surrounding said plurality of coaxial cables, theimprovement comprising a sheath comprising, in combination: A. a layerof hygroscopic material surrounding said plurality of coaxial cableinteriorally of said outer jacket and substantially coextensive withsaid plurality of coaxial cables to serve as a sink for absorbingmoisture; and B. a metal shield surrounding said layer of hygroscopicmaterial and disposed between said layer of hygroscopic material andsaid outer jacket, said metal shield having the edges thereof inoverlapping relationship to form a longitudinal seam bonded together bymeans of an adhesive.
 11. A cable according to claim 10 which includes alayer of plastic material disposed between said plurality of coaxialcables and said layer of hygroscopic material.
 12. A cable according toclaim 10 wherein an adhesive comprising a copolymer of an olefin and anethylenically unsaturated carboxylic acid is disposed as a layer oversubstantially the entire areas of contact a. between said secondconductor and said outer jacket in each of said coaxial cables, and b.also between said metal shield and said outer jacket.
 13. A cableaccording to claim 10 wherein a. said insulating means surrounding saidfirst conductor in each of said coaxial cables comprises a foamedpolyolefin, b. said second conductor in each of said coaxial cables isfashioned of a metal selected from the group consisting of aluminum andcopper, and c. said metal shield surrounding said layer of hygroscopicmaterial is fashioned of a metal selected from the group consisting ofaluminum, copper, bronze, steel, and composites of two or more of theforegoing metals.
 14. A cable according to claim 10 wherein said layerof hygroscopic material is fashioned of a material selected from thegroup consisting of paper, cloth, blends of polymer material and calciumchloride, blends of polymer material and sodium sulfate, and blends ofpolymer material and sodium chloride.
 15. In a twisted pair cablecomprising a pair of conductors individually insulated with a layer offoamed polyolefin and twisted together to form a twisted pair havingopposed helical valleys defined by the outer surfaces of said layers offoamed polyolefin, a layer of foamed polyolefin surrounding said twistedpair and disposed substantially throughout the helical valleys therein,a protective metal shield surrounding said layer of foamed polyolefinand characterized by having a longitudinal seam formed by overlappingthe edges of a metal strip used to form said protective metal shield,and an outer jacket of plastic material, the improvement comprising asheath comprising, in combination: A. a layer of hygroscopic materialdisposed around said protective metal shield interiorally of said outerjacket and substantially coextensive with said protective metal shieldto serve as a sink for absorbing moisture; and B. a metal shieldsurrounding said layer of hygroscopic material and disposed between saidlayer of hygroscopic material and said outer jacket, said metal shieldhaving the edges thereof in overlapping relationship to form alongitudinal seam bonded together by means of an adhesive.
 16. A twistedpair cable according to claim 15 which includes a layer of plasticmaterial disposed between said protective metal shield and said layer ofhygroscopic material.
 17. A twisted pair cable according to claim 15wherein said layer of hygroscopic material is fashioned of a materialselected from the group consisting of paper, cloth, blends of polymermaterial and calcium chloride, blends of polymer material and sodiumsulfate, and blends of polymer material and sodium chloride.
 18. Atwisted pair cable according to claim 15 wherein an adhesive comprisinga copolymer of an olefin and an ethylenically unsaturated carboxylicacid is disposed as a layer over substantially the entire areas ofcontact between a. said layer of foamed polyolefin surrounding saidtwisted pair and said protective metal shield, and b. also between saidmetal shield and said outer jacket.